1. Field of the Invention
The present invention relates to a liquid ejection head. More particularly, the present invention relates to a liquid ejection head that can suitably be utilized in the technological field of inkjet recording.
2. Description of the Related Art
Recording apparatus equipped with a liquid ejection head have recently been and are currently being used not only for home printer applications but also for business printer applications including commercial printer applications and retail photo printer applications. In short, the demand for such recording apparatus is expanding. High speed/high image quality recording performances are required to liquid ejection heads to be used for business printer applications. To meet the requirement, line heads that are liquid ejection heads having a width greater than the width of the recording mediums to be used with the liquid ejection head have been proposed and are getting popularity. In a line head, a large number of ejection ports from which liquid is ejected are arranged highly densely than ever. In general, a line head is formed by arranging a plurality of short recording element substrates on a base substrate having a considerable length.
Some line heads are formed by using a plurality of recording element substrates that adopt a thermal system or a shear-mode piezo system as liquid ejection system. As such a line head is driven for a high speed recording operation, the line head generates heat to a large extent so that the temperature of the recording element substrates is apt to rise high. As the temperature of the recording element substrates rises, the temperature of the liquid contained in the inside also rises to change the viscosity of the liquid to by turn change the quantity of liquid droplets that the line head ejects in the same image recording operation. In this way, the ejection characteristics of the line head are affected by temperature changes. Additionally, temperature differences can arise among the recording element substrates. Generally, liquid is supplied to each of the recording element substrates through a common flow channel that is formed within the head. Then, liquid that is heated at the upstream side flows down to the downstream side to give rise to temperature differences among the recording element substrates. Such temperature differences by turn can result in an image that represents irregularities in the width direction. When the temperature of a single recording element substrate is forced to fluctuate with time to a large extent, on the other hand, the produced image can represent irregularities in the recording medium feeding direction. Commercial printer applications require a high recording speed and an image quality above a certain quality level at the same time. Therefore, how to reduce such temperature differences of liquid is an important problem that needs to be dissolved.
Japanese Patent Publication No. 4,729,957 describes a line head including spacer members arranged on a base substrate so as to support respective recording element substrates. Each of the spacer members has a liquid chamber formed in the inside thereof. The spacer members are provided for the purpose of improving the easiness of replacing defective recording element substrates and absorbing the differences in the thickness among some component members. When the structure of such a line head is examined from the viewpoint of heat emission, the heat emitted from each of the recording element substrates is less easily conducted to the base substrate because of the spacer member interposed between the recording element substrate and the base substrate. Therefore, thermal interferences among the recording element substrates via the base substrate are suppressed. Thus, the temperature of each of the recording element substrates does not depend on the position where it is arranged on the base substrate but depends on the ratio of the quantity of heat it generates to the quantity of liquid it ejects, its printing duty and its temperature control means, which may typically be so-called sub-heaters. Then, temperature differences seldom arise among the recording element substrates so that image irregularities in the width direction will effectively be suppressed.
However, with the arrangement described in Japanese Patent Publication No. 4,729,957, when a recording element substrate is subjected to a temperature control operation by means the temperature control means thereof, which may typically be sub-heaters, in a recording standby status, for example, the temperature of the recording element substrate transitionally rises at the time of starting an image recording operation. Then, as a result, image irregularities arise immediately after the start of the recording operation. This is because the temperature of the liquid in the liquid chamber in the corresponding spacer member is raised by the heat generated by the temperature control means in a recording standby status during the temperature control operation so that consequently the heated liquid is supplied to the recording element substrate when the recording operation is started. Such a transitional temperature rise does not take place if no temperature control operation is conducted in a recording standby status. However, in the case of thermal systems and shear-mode piezo systems, the temperature of a recording element substrate can get to 50° C. in a high duty continuous image recording operation. Therefore, temperature control in a recording standby status is necessary because otherwise the temperature rise at the time of starting an image recording operation is so high as to give rise to image irregularities immediate after the start of the recording operation.